What
---
Handle [ProtocolError](https://cs.opensource.google/go/go/+/master:src/net/textproto/textproto.go;drc=ee6b34797b66b957fdf9b7211ee3f0b80ac57758;l=46) for multipart upload.

Why
---
When a user sends an invalid `Content-Disposition` Go sends a [malformed
MIME header
line](https://cs.opensource.google/go/go/+/master:src/net/textproto/reader.go;l=526?q=%22malformed%20MIME%20header%20line%22&ss=go%2Fgo)
and we end up returning a 500 error instead of a 400 error.

Since this is an invalid value that our user sends we need to return a
400 instead of 500, so it doesn't attribute to our error rate, and
to indicate the user did something wrong.

Reference: https://gitlab.com/gitlab-org/gitlab/-/issues/432533


Changelog: fixed
Signed-off-by: Steve Xuereb <sxuereb@gitlab.com>